Yeah, the whole Wanko romance aspect feels like waste of time, at least in any other way but giving her a solid background and personality. Rea and Chihiro are bound together in far too many ways. Rea's personality is still somewhat unknown, so would she let Chihiro live the rest of his life with a zombie if there's a living girl waiting? An undead creature can't very well give birth, so while she can experience what a normal girl could, she won't be able to experience everything a normal woman could.